Slide gates are used for the casting openings of steelcasting ladles. The slide gate is closed when the molten steel is poured into the ladle and is opened for casting of the molten metal.
A slide gate comprises a refractory head plate having an opening for passage of the molten metal and a refractory slide plate beneath the head plate and also having an opening so that by sliding the plate one way or another the head plate opening is blocked or opened. The slide plate is positioned and slidably guided by guideways fixed to the ladle bottom. The interfacing surfaces of the two refractory plates are flat to make the sliding action possible. The two refractory plates, which may be made of ceramic material, are directly contacted by the molten metal and thereby heated to the metal's casting temperature.
The prior art includes a proposal that one or both of the refractory plates be impregnated with tar to provide the following advantages:
1. Reduction of the infiltration of slag and steel;
2. Improvement of the closing behavior and sliding behavior as a result of sperficially vaporizing tar constituents; and
3. Improvement of the spalling resistance of the ceramic material, for example a reduced tendency to form cracks.
However, there are objectionable features. The vaporized tar is driven off in excessive volumes and condenses on the parts of the slide gate so that with objectionable frequency the gate must be dismantled and cleaned from the condensed tar. This is an unpleasant task for the workman and prevents use of the ladle until the slide gate can be reassembled and again ready for use. In addition, a redundant amount of tar is involved, the result being dirty plumes of tar vapor which rise and present an environmental problem to the workmen.
The slide plate is in its closed position when the molten steel is poured in the ladle and is subjected to a large and sudden temperature change. Temperature measurements have shown that there is a temperature gradient of from 1600.degree. C. to about 300.degree. C. over the length of the slide plate. The head plate presumably is subjected to a corresponding temperature gradient throughout its length. As a consequence, the refractory ceramic plates receive high thermal strains and these strains have caused spider-like cracks in the plates.
The object of the present invention has been to develop a more satisfactory slide and/or head plate for such a molten metal slide gate and which provides the advantages of the use of the tar while eliminating or at least materially reducing the tar's disadvantages as well as the development of the spider-like cracks in the refractory plates.